Human immunodeficiency virus protease inhibitors. From drug design to clinical studies

Precursors for cytochrome P450 profiling breath tests from an in silico screening approach

The family of cytochrome P450 enzymes (CYPs) is a major player in the metabolism of drugs and xenobiotics. Genetic polymorphisms and transcriptional regulation give a complex patient-individual CYP activity profile for each human being. Therefore, personalized medicine demands easy and non-invasive measurement of the CYP phenotype. Breath tests detect volatile organic compounds (VOCs) in the patients' exhaled air after administration of a precursor molecule. CYP breath tests established for individual CYP isoforms are based on the detection of (13)CO2 or (14)CO2 originating from CYP-catalyzed oxidative degradation reactions of isotopically labeled precursors.We present an in silico work-flow aiming at the identification of novel precursor molecules, likely to result in VOCs other than CO2 upon oxidative degradation as we aim at label-free precursor molecules. The ligand-based work-flow comprises five parts: (1) CYP profiling was encoded as a decision tree based on 2D molecular descriptors derived from established models in the literature and validated against publicly available data extracted from the DrugBank. (2) Likely sites of metabolism were identified by reactivity and accessibility estimation for abstractable hydrogen radical. (3) Oxidative degradation reactions (O- and N-dealkylations) were found to be most promising in the release of VOCs. Thus, the CYP-catalyzed oxidative degradation reaction was encoded as SMIRKS (a programming language style to implement reactions based on the SMARTS description) to enumerate possible reaction products. (4) A quantitative structure property relation (QSPR) model aiming to predict the Henry constant H was derived from data for 488 organic compounds and identifies potentially VOCs amongst CYP reaction products. (5) A blacklist of naturally occurring breath components was implemented to identify marker molecules allowing straightforward detection within the exhaled air.Evident oxidative degradation reactions served as test case for the screening approach. Comparisons to metabolism data from literature support the results' plausibility. Thus, a large scale screening for potential novel breath test precursor using the presented five stage work-flow is promising.

Liquid chromatography tandem mass spectrometric studies of indinavir sulphate and its forced degradation products

Indinavir sulphate was subjected to forced degradation under hydrolysis (acidic, basic and neutral), oxidation, photolysis and thermal stress as prescribed by ICH guidelines. It was degraded under acidic, basic, neutral and oxidative stress conditions, while it was stable under other conditions. After degradation total eight degradation products were formed. The degradation products were identified and their separation was accomplished on Waters XTerra(®) C(18) column (250 mm × 4.6mm i.d., 5 μm) using 20mM ammonium actate:acetonitrile as (50:50, v/v) mobile phase in an isocratic elution mode by LC. The method was extended to LC-MS/MS for characterization of the degradation products and the fragmentation pathways were proposed. The proposed structures of degradation products were also confirmed by HRMS studies. No previous reports were found in the literature regarding the characterization of degradation products of indinavir sulphate.

Analysis of the pharmacokinetic boosting effects of ritonavir on oral bioavailability of drugs in mice

  Ritonavir dramatically increases the bioavailability of a variety of concurrently administered drugs by inhibition of metabolic enzymes and drug transporters. The purpose of this study was to investigate the extent to which ritonavir's inhibition of drug transporters and/or CYP3A contributes to the increased oral bioavailability in mice. The area under the plasma concentration-time curves (AUC) for orally administered saquinavir after coadministration with 50 mg/kg ritonavir dramatically increased (325-fold). As a result, the bioavailability, Fa·Fg and Fh increased 75-, 38- and twofold, respectively. In addition, the increase in the AUC predicted from the in vitro Ki value was ninefold, which was derived from the inhibition of metabolic enzymes by ritonavir in the liver. The remaining 36-fold increase in the AUC was considered to be derived from the inhibition in the small intestine. The AUCinf for probe substrate midazolam, fexofenadine, and pravastatin increased after the oral administration of ritonavir by only five-, 13-, and sevenfold, respectively. Moreover, the AUC0-12 for saquinavir was affected negligibly by itraconazole. These results indicate ritonavir mainly affects the first-pass effect of saquinavir in the small intestine, increasing the bioavailability of orally administered saquinavir. Furthermore, cyp isoforms other than CYP3A, which contribute to the metabolism of saquinavir in humans, are involved in the metabolism of saquinavir in mice.

Opportunities and challenges for oral delivery of hydrophobic versus hydrophilic peptide and protein-like drugs using lipid-based technologies

Peptide and protein-like drugs are macromolecules currently produced in increasing numbers by the pharmaceutical biotechnology industry. The physicochemical properties of these molecules pose barriers to oral administration. Lipid-based drug-delivery systems have the potential to overcome these barriers and may be utilized to formulate safe, stable and efficacious oral medicines. This review outlines the design of such lipid-based technologies. The mechanisms whereby these formulations enhance the absorption of lipophilic versus hydrophilic peptide and protein-like drugs are discussed. In the case of lipophilic compounds, the advantages of lipid-based drug-delivery systems including increased solubilization, decreased intestinal efflux, decreased intracellular metabolism and possible lymphatic transport are well established as is evident from the success of Neoral® and other drug products on the market. In contrast, with respect to hydrophilic compounds, the situation is more complex and, while promising formulation approaches have been studied, issues including reproducibility of response, intersubject variability and duration of response require further optimization before commercially viable products are possible.

Influence of drug transport proteins on the pharmacokinetics and drug interactions of HIV protease inhibitors

Protease inhibitors, a class of antiretroviral agents frequently used in the treatment of HIV infection, interact with numerous transport proteins resulting in clinically significant drug-drug interactions (DDIs). This review focuses on the proteins that transport protease inhibitors and directly influence the pharmacokinetics of these drugs, as well as the transport proteins that are inhibited or induced by protease inhibitors. Clinically relevant DDIs involving drug transporters and protease inhibitors, either as "victim" drugs or as "perpetrator" drugs, and the pharmacokinetic consequences of such interactions are highlighted. A summary of transporter-mediated processes underlying the toxicity of protease inhibitors is provided. Finally, the effect of HIV infection or co-infection on drug transport proteins, and the implications for protease inhibitor pharmacokinetics is discussed. Transport proteins significantly influence the pharmacokinetics, efficacy and toxicity profiles of this important class of drugs.

Prediction of in vivo intestinal absorption enhancement on P-glycoprotein inhibition, from rat in situ permeability

The purpose of this study is to determine the functional role of P-glycoprotein (P-gp) in intestinal absorption of drugs and to quantitatively predict the in vivo absorption enhancement on P-gp inhibition. In situ single-pass rat ileum permeability and aqueous solubility were measured for a set of 16 compounds. Permeability studies were also carried out in the presence of P-gp inhibitor to estimate the permeability enhancement on P-gp inhibition. A significant correlation was obtained between rat ileum permeability and the literature human intestinal absorption (HIA), F(a,human) (r = 0.891; p < 0.01). Compounds with permeability >0.2 x 10(-4) cm/s are completely absorbed; however, few practically insoluble compounds were overestimated with this relationship. Inhibition of P-gp increased the permeability (p < 0.05) of three moderately and three highly permeable compounds. Efflux inhibition ratio (EIR), the ratio of permeability due to P-gp-mediated efflux activity and passive permeability only, for these compounds was in the order of digoxin > paclitaxel > fexofenadine > quinidine > verapamil > cyclosporine. Integration of EIR with permeability versus F(a,human) predicted that modulation of P-gp has no significant effect on the absorption of highly permeable compounds (quinidine, verapamil, and cyclosporine A), while for moderately permeable compounds (digoxin, paclitaxel, and fexofenadine), P-gp profoundly influences the intestinal permeability. The in situ permeability in rat ileum may be used to predict the in vivo P-gp function and its quantitative contribution to intestinal drug absorption. Integration of the functional activity of P-gp with the characteristics of BCS may explain drug interactions and explore the possible pharmacokinetic advantage on P-gp inhibition.

Interaction with indinavir to enhance systemic exposure of an investigational HIV protease inhibitor in rats, dogs and monkeys

1. The use of a beneficial interaction between indinavir and compound A, a potent investigational HIV protease inhibitor to enhance systemic exposure of compound A, was investigated. 2. When administrated alone, compound A underwent extensive hepatic first-pass metabolism in rats and monkeys, resulting in low oral bioavailability. 3. In vitro studies with liver microsomes revealed that compound A metabolism was mediated exclusively by CYP3A enzymes in rats, dogs and monkeys. Indinavir, which also was metabolized predominantly by CYP3A enzymes, extensively inhibited compound A metabolism in microsomes, whereas compound A showed weak inhibitory potency on indinavir metabolism. 4. Consistent with in vitro observations, co-administration of the two compounds resulted in a 17-fold increase in oral AUC of compound A in rats owing to the inhibition of metabolism of compound A by indinavir, whereas compound A did not affect indinavir metabolism as indicated by the unchanged indinavir AUC. Similarly, the systemic exposure of compound A in dogs and monkeys was increased substantially following oral co-administration with indinavir by 7- and > 50-fold, respectively. 5. Enhancement in compound A systemic exposure by indinavir in humans, as predicted based on the in vivo animal and in vitro human liver microsomal data, was confirmed in subsequent clinical studies.

Drug-drug interaction mediated by inhibition and induction of P-glycoprotein

P-glycoprotein (P-gp), the most extensively studied ATP-binding cassette transporter, functions as a biological barrier by extruding toxic substances and xenobiotics out of cells. In vitro and in vivo studies have demonstrated that P-gp plays a significant role in drug absorption and disposition. Like cytochrome P450 enzymes, inhibition and induction of P-gp have been reported as the causes of drug-drug interactions. Because many prototypic inhibitors and inducers affect both CYP3A4 and P-gp, many drug interactions caused by these inhibitors and inducers involve these two systems. Clinically, it is very difficult to quantitatively differentiate P-gp-mediated drug interactions versus CYP3A4-mediated drug interactions, unless their relative contributions can be accurately estimated. Therefore, care should be exercised when interpreting drug interaction data and exploring the underlying mechanisms of drug interactions.

Barriers to Alzheimer disease drug discovery and development in academia

The drug discovery and the drug development processes represent a continuum of recursive activities that range from initial drug target identification to final Food and Drug Administration approval and marketing of a new therapeutic. Drug discovery, as its name implies, is more exploratory and less focused in many cases, whereas drug development has a clinically defined endpoint and a specific disease goal. Academia has historically made major contributions to this process at the early discovery phases. However, current trends in the organization of the pharmaceutical industry suggest an expanded role for academia in the near future. Megamergers among major pharmaceutical corporations indicate their movement toward a focus on end-stage clinical trials, manufacturing, and marketing. There has been a parallel increase in outsourcing of intermediate steps to specialty small pharmaceutical, biotechnology, and contract service companies. The new paradigm suggests that academia will play an increasingly important role at the proof-of-principle stage of basic and clinical drug discovery research, in training the future skilled work force, and in close partnerships with small pharmaceutical and biotechnology companies. However, academic drug discovery research faces a set of barriers to progress, the relative importance of which varies with the home institution and the details of the research area. These barriers fall into four general categories: (1) the historical administrative structure and environment of academia; (2) the structure and emphasis of peer review panels that control research funding by government and private agencies; (3) the organization and operation of the academic infrastructure; and (4) the structure and availability of specialized resources and information management. Selected examples of barriers to drug discovery and drug development research and training in academia are presented, as are some specific recommendations designed to minimize or circumvent these barriers. In some cases, precedents established by other disease-focused areas may be relevant to Alzheimer disease and related disorders, but the overall impact of any changes requires adaptation at the top of the administrative structures in academia and funding agencies to support and encourage cooperative efforts among faculty investigators.

Structure-based design and synthesis of HIV-1 protease inhibitors employing beta-D-mannopyranoside scaffolds

A preliminary account on the structure-based design, synthesis and evaluation of peptidomimetic inhibitors of HIV-1 protease containing beta-D-mannopyranoside scaffolds is given. The compounds prepared had IC(50) values in the micromolar range. The results provide a platform for the development of more potent carbohydrate-based inhibitors of HIV-1 and other aspartic proteases.

A modified coumarinic acid-based cyclic prodrug of an opioid peptide: its enzymatic and chemical stability and cell permeation characteristics

PURPOSE

To evaluate the chemical/enzymatic stability and the cell permeation characteristics of the modified coumarinic acid-based cyclic prodrug 2 of DADLE (H-Tyr-D-Ala-Gly-Phe-D-Leu-OH), which has an aldehyde equivalent (oxymethyl) inserted between the phenolic group of the promoiety and the carboxylic acid group of the peptide.

METHODS

The rates of the chemical/enzymatic conversion of the oxymethyl-modified prodrug 2 to DADLE were measured by HPLC. The cellular permeation characteristics of DADLE and its oxymethyl-modified prodrug 2 were measured by HPLC using Caco-2 cells, wild type Madin-Darby Canine Kidney cells (MDCK-WT), MDCK cells transfected with human MDR1 gene (MDCK-MDR1), and MDCK cells transfected with human MRP2 gene (MDCK-MRP2) grown onto microporous membranes.

RESULTS

The oxymethyl-modified coumarinic acid-based cyclic prodrug 2 degraded chemically to DADLE in a pH-dependent manner, i.e., rates of conversion increased with increasing pH. The prodrug 2 degraded rapidly in rat plasma (t1/2 = 39 min) and rat liver homogenate (t1/2 = 59.2 min), but much slower in Caco-2 cell homogenate (t1/2 = 678.7 min) and human plasma (t1/2 = 264.3 min). In all four cell lines used for transport studies, the flux rates of the oxymethyl prodrug 2 in the basolateral (BL)-to-apical (AP) direction (Papp BL-to-AP) were significantly greater than the flux rates in the AP-to-BL direction (Papp AP-to-BL). The Papp BB-to-AP/Papp AP-to-BL ratios were >116, 35.1, 21.2, and 12.6 in Caco-2, MDCK-MDR1, MDCK-MRP2, and MDCK-WT cells, respectively. The efflux of the modified prodrug could be inhibited by GF120918 (an inhibitor for P-gp) and cyclosporin A (an inhibitor for P-gp and MRP2).

CONCLUSIONS

The oxymethyl-modified coumarinic acid-based cyclic prodrug 2 of DADLE could be converted to DADLE in both chemical and enzymatic media. However, the prodrug was a good substrate for both P-gp and MRP2 suggesting that its permeation across intestinal mucosa and blood-brain barrier would be significantly restricted.

Antiviral activity of UIC-PI, a novel inhibitor of the human immunodeficiency virus type 1 protease

The human immunodeficiency virus type 1 (HIV-1) protease inhibitor UIC-PI (1) was developed via structure-based design and incorporated a novel bis-tetrahydrofuran (bis-THF) ligand in the (R)-(hydroxyethyl)sulfonamide based isostere. The EC(50) and EC(90) of the compound in acutely-infected H9 cells were <1 and approximately 1 nM, respectively. In chronically infected H9/HIV-1(IIIB) cells, the EC(50) and EC(90) were 20 and 50 nM, respectively. In parallel studies comparing UIC-PI and saquinavir in H9/HIV-1(IIIB) cells, viral p24 levels in culture supernatants were an order of magnitude lower with UIC-PI than with saquinavir.

HIV protease inhibitors attenuate adherence of Candida albicans to epithelial cells in vitro

Oropharyngeal candidiasis is one of the first and most commonly reported opportunistic infections of untreated AIDS patients. With the introduction of the new antiviral HAART therapy, including HIV protease inhibitors, this mucocutaneous infection is nowadays only rarely observed in treated patients. It was recently shown that HIV protease inhibitors have a direct attenuating effect on Candida albicans secreted aspartic proteinases (Saps), an investigation prompted by the fact that both Sap and HIV protease belong to the superfamily of aspartic proteinases and by the observation that mucocutaneous infections sometimes resolve even in the absence of an immunological improvement of the host. As these Saps are important fungal virulence factors and play a key role in adhesion to human epithelial cells we tried to assess the effect of the HIV protease inhibitors Ritonavir, Indinavir and Saquinavir on fungal adhesion to these cells. The effect on phagocytosis by polymorphonuclear leukocytes was also assessed. Ritonavir was found to be the most potent inhibitor of fungal adhesion. A dose-dependent inhibition of adhesion to epithelial cells was found already at 0.8 microM and was significant at 4 microM or higher, at 500 microM the inhibition was about 55%. Indinavir and Saquinavir inhibited significantly at 4 microM or 20 microM, respectively; at 500 microM the inhibition was 30% or 50%. In contrast, no protease inhibitor was able to modulate phagocytosis of Candida by polymorphonuclear leukocytes. In conclusion, inhibition of Saps by HIV protease inhibitors may directly help to ease the resolution of mucosal candidiasis. In future, derivatives of HIV protease inhibitors, being more specific for the fungal Saps, may form an alternative in the treatment of mucosal candidiasis insensitive to currently available antimycotics.

The influence of donor and reservoir additives on Caco-2 permeability and secretory transport of HIV protease inhibitors and other lipophilic compounds

PURPOSE

To optimize the conditions for determining Caco-2 permeation of HIV protease inhibitors and other lipophilic compounds, and to compare cyclic urea HIV protease inhibitors with marketed compounds.

METHODS

Absorptive and secretory Caco-2 membrane permeation studies were performed with HIV protease inhibitors and various reference compounds, examining the effects of adding the solubilizing agents dimethylacetamide (DMAC) and albumin in donor and reservoir compartments, respectively.

RESULTS

DMAC was useful as an additive in the donor vehicles, increasing the dissolved concentrations of poorly water-soluble HIV protease inhibitors, and enabling more reliable determination of P(app) values. Donor vehicles containing up to 5% DMAC could be used without altering Caco-2 barrier function, as indicated by the lack of effect on permeabilities of reference compounds with diverse absorption characteristics. The utilization of a reservoir containing albumin resulted in marked increases in absorptive Papp values for some HIV protease inhibitors as well as other lipophilic, highly protein bound compounds, consistent with albumin increasing the release of these compounds from the cell monolayer.

CONCLUSIONS

Poorly soluble, lipophilic, highly bound compounds may require using solubilizing agents in the donor and reservoir compartments of Caco-2 permeation experiments for estimating in vivo absorption potential. If the reservoir does not provide adequate sink conditions, cellular retention could over-emphasize the contributions of secretory transport. The cyclic ureas, DMP 450, DMP 850, and DMP 851, have Caco-2 permeabilities suggestive of moderate-to-high oral absorption potential in humans.

Oral absorption of the HIV protease inhibitors: a current update

Although the human immunodeficiency virus (HIV) protease inhibitors are highly effective, they are characterized by low and/or variable bioavailability with limited penetration into the central nervous system (CNS). Their clinical use is limited by patient compliance and by drug-drug interactions. The effect of drug solubility on their oral absorption has been investigated but further evaluation of this relationship is required. First pass metabolism appears to be significant for the HIV protease inhibitors and they are extensively metabolized by cytochrome P450 (CYP) 3A4. Recent studies suggest that these drugs are substrates for the P-glycoprotein efflux pump, which can limit their intestinal absorption and their transport across the blood-brain barrier. Drugs inducing or inhibiting CYP3A4 and/or P-glycoprotein may influence the bioavailability of the HIV protease inhibitors. The low bioavailability, variable absorption and drug-drug interactions of the HIV protease inhibitors may be related to the variability of cytochrome P450 and P-glycoprotein expression and to possible CYP3A4/P-glycoprotein interactions. To improve oral HIV protease inhibitor therapy, it is essential to mechanistically characterize the cell specific, tissue specific and regional intestinal dependencies of drug transport, secretory transport, metabolism and P-glycoprotein/CPY3A4 interactions. This report reviews the physicochemical characteristics and pharmacokinetics of the HIV protease inhibitors while considering the relationships between their hepatic and intestinal metabolism, low bioavailability, variable absorption and drug-drug interactions.

P-glycoprotein, secretory transport, and other barriers to the oral delivery of anti-HIV drugs

Orally administered anti-HIV drugs must be adequately and consistently absorbed for therapy to be successful. This review discusses the barriers to achieving oral bioavailability for the currently available anti-HIV drugs. Most reverse transcriptase inhibitors have good oral bioavailabilities. Didanosine bioavailability could be reduced by acid instability, first-pass hepatic metabolism, and possibly poor intestinal permeation. Bioavailability of zidovudine is also reduced by first-pass metabolism. The non-nucleoside reverse transcriptase inhibitors have oral bioavailabilities most probably limited by poor aqueous solubility. For each of the currently marketed HIV protease inhibitors, solubility, intestinal permeability, and first-pass metabolism could contribute to reducing oral bioavailability. The intestinal permeabilities of these agents is influenced by secretory transport. In vitro, secretory transport, which appears to be P-glycoprotein-mediated, is much greater than permeation in the absorptive direction for indinavir, nelfinavir, ritonavir, and saquinavir. The mechanisms of secretory intestinal transport are reviewed, and the factors that may influence the impact of secretory transport in vivo are considered.

Pharmacokinetic interactions between HIV-protease inhibitors in rats

The interactions of four HIV-protease inhibitors, ritonavir (RIT), saquinavir (SAQ), indinavir (IND) and nelfinavir (NEL), were examined by in vitro metabolic studies using rat liver microsomal fractions. The substrate concentrations employed were 0.75 approximately 12 microM, and the inhibitor concentrations were 2.5 approximately 60 microM. The metabolic clearance rates of SAQ, NEL and IND as determined by V(max)/K(m) were 170.9+/-10.9, 126.0+/-4.4 and 73.0+/-2.0 microL/min/mg protein, respectively. RIT was a potent inhibitor of the other three protease inhibitors, and the inhibition constants (K(i)) were 1.64 microM for SAQ, 0.95 microM for IND and 1. 01 microM for NEL. NEL was the second strongest inhibitor with a K(i) for NEL inhibition of IND metabolism of 2.14 microM. IND was the third strongest inhibitor with K(i)s of 2.76 microM for inhibition of NEL and 3.55 microM for inhibition of SAQ. As SAQ has the highest metabolic clearance rate, the K(i) for the SAQ inhibition of IND metabolism was high, 9.50 microM. Based on these in vitro results, drug interactions between NEL and IND or RIT were studied after oral administration to rats where the dose of each drug was 20 mg/kg. The C(max) and AUC of NEL were increased 3.6- and 8.5-fold by the co-administration with RIT. However, in contrast to co-administration of NEL and RIT, the effect of IND on the pharmacokinetics of NEL was negligible and the t(1/2) of NEL was not significantly increased by IND. Therefore, the combination of NEL and IND is recommended as a combination therapy for AIDS patients.

Tricyclic ureas: a new class of HIV-1 protease inhibitors

A new class of tricyclic ureas containing a conformationally constrained proline was designed with the aid of molecular modeling. Efficient stereoselective intermolecular pinacol coupling represented the highlight of the synthesis. These rigid cyclic ureas are active towards HIV-1 protease, with 9 being the most potent compound (Ki = 9 nM) despite interacting with only three side chain binding pockets of HIV protease.

Inhibition and induction of cytochrome P450 and the clinical implications

The cytochrome P450s (CYPs) constitute a superfamily of isoforms that play an important role in the oxidative metabolism of drugs. Each CYP isoform possesses a characteristic broad spectrum of catalytic activities of substrates. Whenever 2 or more drugs are administered concurrently, the possibility of drug interactions exists. The ability of a single CYP to metabolise multiple substrates is responsible for a large number of documented drug interactions associated with CYP inhibition. In addition, drug interactions can also occur as a result of the induction of several human CYPs following long term drug treatment. The mechanisms of CYP inhibition can be divided into 3 categories: (a) reversible inhibition; (b) quasi-irreversible inhibition; and (c) irreversible inhibition. In mechanistic terms, reversible interactions arise as a result of competition at the CYP active site and probably involve only the first step of the CYP catalytic cycle. On the other hand, drugs that act during and subsequent to the oxygen transfer step are generally irreversible or quasi-irreversible inhibitors. Irreversible and quasi-irreversible inhibition require at least one cycle of the CYP catalytic process. Because human liver samples and recombinant human CYPs are now readily available, in vitro systems have been used as screening tools to predict the potential for in vivo drug interaction. Although it is easy to determine in vitro metabolic drug interactions, the proper interpretation and extrapolation of in vitro interaction data to in vivo situations require a good understanding of pharmacokinetic principles. From the viewpoint of drug therapy, to avoid potential drug-drug interactions, it is desirable to develop a new drug candidate that is not a potent CYP inhibitor or inducer and the metabolism of which is not readily inhibited by other drugs. In reality, drug interaction by mutual inhibition between drugs is almost inevitable, because CYP-mediated metabolism represents a major route of elimination of many drugs, which can compete for the same CYP enzyme. The clinical significance of a metabolic drug interaction depends on the magnitude of the change in the concentration of active species (parent drug and/or active metabolites) at the site of pharmacological action and the therapeutic index of the drug. The smaller the difference between toxic and effective concentration, the greater the likelihood that a drug interaction will have serious clinical consequences. Thus, careful evaluation of potential drug interactions of a new drug candidate during the early stage of drug development is essential.

In vitro and in vivo drug interactions involving human CYP3A

Cytochrome P4503A (CYP3A) is importantly involved in the metabolism of many chemically diverse drugs administered to humans. Moreover, its localization in high amounts both in the small intestinal epithelium and liver makes it a major contributor to presystemic elimination following oral drug administration. Drug interactions involving enzyme inhibition or induction are common following the coadministration of two or more CYP3A substrates. Studies using in vitro preparations are useful in identifying such potential interactions and possibly permitting extrapolation of in vitro findings to the likely in vivo situation. Even if accurate quantitative predictions cannot be made, several classes of drugs can be expected to result in a drug interaction based on clinical experience. In many instances, the extent of such drug interactions is sufficiently pronounced to contraindicate the therapeutic use of the involved drugs.